In the book The Fellowship of the Ring, Glorfindel puts the injured Frodo on his horse. The other hobbits then have trouble keeping up with the horse, Glorfindel, and Strider. That got me wondering how much hobbits weigh, and why Glorfindel didn't put all four of the hobbits on the one horse.

The book also says that hobbits in the Third Age rarely exceeded 3 feet in height. I know elves are skinny, but they are also very tall, so it seems like one elf over 6 feet tall must weigh more than four hobbits under 3 feet tall. So why not let all the hobbits ride the horse together until the Black Riders approach? If the horse was able to escape the riders with Frodo on its back after carrying an elf for a week, it would also be able to escape the riders with Frodo on its back after carrying an elf for a week and 4 hobbits for a day or two.

I don't know that I agree that "one elf over 6 feet tall must weigh more than four hobbits under 3 feet tall". Remember that Elves can walk on snow without denting it, they're supernaturally light and airy. Hobbits are earthy, and oftentimes fat. Even without this distinction, four real-world dwarfs or midgets will weigh much more than 1 kinda-tall man.
– NerrolkenMay 4 '15 at 23:02

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it may not be much, but it must be enough to a horse to notice. Remember that Theoden doesn't let Merry to ride into battle with him because he's too heavy for any of his riders to carry.
– KutuluMikeMay 4 '15 at 23:05

5 Answers
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Merry is too heavy for a man to carry in to battle with him, but not too heavy for a woman (who was wearing full armor). You could extrapolate that a hobbit + a woman in full armor would weight no more than a man in full armor.

Hobbits are short, but so are dwarves. Yet dwarves are quite dense and weighty (though not too heavy to be tossed. Don't tell the Elf.), so height might not be a deterrent to higher weight to height ratio.

Hobbits eat upwards of 6 times a day and are rather plump.

Answer - may never be definitively known, but probably more than a 3 ft tall human. Possibly quite a bit more.

Also, assuming all 4 hobbits are 3 ft tall, that would be 12 ft total in height. FAR more weight than a 6 ft tall elf, even if the elf were chunky (which I doubt).
– InessariaMay 4 '15 at 23:11

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I always interpreted the "we can't carry you" thing as either an outright lie or a truth from people who were sacrificing sandwiches to conserve weight. Riding a hundred miles nonstop at a gallop bearing armored warriors is very different from trotting 20 miles, then galloping half a mile.
– Wad CheberMay 5 '15 at 1:14

As for height/weight ratios, I disagree. I was 13 inches long at birth and weighed a hefty 10 lbs. If ratios were constant, that would mean a nearly 14 foot tall person would weigh 100 lbs. Ever seen a little person (I.e., a midget)? Their proportions are VERY different from normal-height people.
– Wad CheberMay 5 '15 at 1:18

Maybe. I still dont think that a horse could carry 4 hobbits, however thin they might be (which is relative. If the hobbits were plump to begin with, and come from a place where plumpness is the norm, "skinny" to them might still be rather hefty.)
– InessariaMay 9 '15 at 22:01

There was no need for Glorfindel to put all 4 hobbits on his horse. Only Frodo was in danger from the Black Riders. Moreover, doing so would slow Frodo's escape and put him in greater peril.

Glorfindel smiled. 'I doubt very much,' he said, 'if your friends would be in danger if you were not with them! The pursuit would follow you and leave us in peace, I think. It is you, Frodo, and that which you bear that brings us all in peril.

Except the other hobbits couldn't keep up with Glorfindel and Strider on foot and Frodo on the horse, so they slowed Frodo down by not being on the horse with him.
– Wad CheberMay 5 '15 at 1:11

Which means they could have covered that 20 miles faster if the hobbits were all riding, and the 3 hobbits who aren't Frodo could have hopped off when the wraiths showed up just before the last 1/4 mile sprint.
– Wad CheberMay 5 '15 at 1:37

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An even better plan is just to take Frodo to Rivendell with Glorfindel as soon as he shows up while the Riders are miles off. Less drama at the fords.
– OldcatMay 5 '15 at 17:15

Glorfindel's horse Asfaloth would travel faster with less wait to carry (for that matter any horse). If Glorfindel made Asfaloth carrying 4 Hobbits, the chances of them being captured by wraiths would have increased.

You must remember the fact that Glorfindel did not travel with Frodo to provide them with better chance of out running the wraiths.

Glorfindel comments that

my horse will not let any rider fall that I command him to bear.

Glorfindel wanted to ensure that Frodo would reach Rivendell safely. Aragorn and Glorfindel could ensure the safety of the other hobbits, just incase wraiths turned on them after failing to capture Frodo.

Tolkien wrote that hobbits were there to four feet tall, and modern hobbits said that they ere shorter than their ancestors in the old days. Thus modern hobbit should be closer to three feet tall and Hobbits in LOTR should have been closer to four feet tall - I believe Tolkien said illustrators should make them about three and a half feet tall in a letter.

I remember a time in my childhood when I was four feet tall and weighed sixty pounds. Four of me would have weighed two hundred and forty pounds. But as a child my legs would have been shorter compared to my torso than those of a four foot tall adult. Assuming that a man six feet tall weighs one hundred eighty pounds an identically proportioned man four feet tall would weigh about 53.333 pounds and an identically proportioned man three feet tall would weigh about 22.5 pounds. Thus four hobbits should weigh about ninety to two hundred thirteen pounds

To estimate the weights of fictional little people, the only factual starting point is what real little people weigh. In a letter describing hobbits for illustration, Tolkien stated that he imagined hobbits as "stout, and short of leg." Achondroplasia, the most common form of dwarfism, seems to be the closest comparison that we have, proportionally speaking, since it has both features described by Tolkien, although it has other typical features as well. According to height/weight charts for achondroplasia, a little person weighs about 35 lbs at 3 ft, 45 lbs at 3 1/2 ft, and 55 lbs at 4 ft.

More pertinent to the context of your question: It's not mentioned that Sam or Frodo are significantly shorter than Merry or Pippin, who top 4 ft after drinking the Ent-draft, so it's reasonable to assume that they were all over 3 1/2 ft, possibly closer to 4 ft, at the beginning of the story. For the sake of convenience, we could estimate 50 lbs each, and that's without their clothes and gear, which could add an additional 1/3 pretty easily, based on gear for a long camping trip. So the four hobbits and their gear would be about 267 lbs.

A fine-boned male elf probably wouldn't weigh more than a slim human male of the same height. According to the BMI index for men, a 6' human male can weigh as little as 140 lbs while still being healthy and a 6'3" human male, as little as 150 lbs. Glorfindel is not described as carrying any gear, so with his clothing, he might weigh 160 lbs.

Horses can carry about 20% of their body weight without increasing heart rate and muscle soreness. Based on the description of Glorfindel's horse, it's more of a racehorse than a draft horse. Arabians and thoroughbreds weigh about 950 and 1000 lbs, respectively, so Glorfindel's horse could carry 190-200 lbs without additional fatigue.

Based on those estimates, carrying all four hobbits would have caused additional fatigue. Since the horse would be unable to carry all four hobbits, carrying additional riders--for example, swapping out--wouldn't have provided any benefit to the overall speed of the party because at least one hobbit would be walking at all times.